Spun yarns of synthetic staple fibers have been produced by cutting continuous filaments into staple fibers, which are then assembled into individual yarn in the same manner as fibers of cotton or wool. A simpler direct spinning process is also used wherein parallel continuous filaments are stretch-broken and drafted between input rolls and delivery rolls in what is sometimes called a stretch break zone or a draft cutting zone to form a sliver of discontinuous fibers which is thereafter twisted to form a spun yarn as disclosed, for example, in U.S. Pat. No. 2,721,440 to New or U.S. Pat. No. 2,784,458 to Preston. Such early processes were slow due to the inherent speed limitations of a true twisting device. As an alternative to true twisting, Bunting et al in U.S. Pat. No. 3,110,151 discloses consolidating staple fibers to make a yarn product using an entangling, or interlacing, jet device for entangling into yarn. Such a product can be produced faster than true twisting, but is not comparable to conventional spun yarns in strength, cleanness, and uniformity. Alternatively, U.S. Pat. No. 4,080,778 to Adams et al discloses a process where a 1500–5000 denier tow of continuous filaments may be heated and drawn, and is then stretch-broken and drafted in a single zone and exits at high speed through an apertured draft roll and an aspirator to maintain co-current flow of fluid and fiber through the roll nip. The discontinuous, unconsolidated filaments are then consolidated in an entangling jet of a type disclosed in Bunting to make a yarn of 50–300 denier. Static charges are removed in the stretch-breaking and drafting zone to minimize splaying. Static removal devices are also placed adjacent the roll pairs that forward the filaments through the process. About 1.5–20% of the discontinuous filaments produced in the stretch-breaking zone exceeds 76 cm in length. The yarn axis is required to be vertical throughout the process. The resultant product is a consolidated yarn with excellent strength, generally higher than ring-spun yarns, which is slub-free and clean.
Multiple stretch-break zones are taught in U.S. Pat. No. 4,924,556 to Gilhaus for progressively reducing the discontinuous filament length for large denier tows which are built up from combining several low weight tows over tensioning guide bars and guiding members. In this way distortions of less than 4.5 can be run with low weight feed tows and production capacity remains high. The combined tows are drawn without breaking in a distortion and heating zone (zone I) at one horizontal level and then passed sequentially through one or more progressively shorter, stretch-breaking zones, (zones II–V) arranged horizontally in another level to conserve floor space. The stretch-breaking zones may comprise one or more “preliminary” breaking zones that progressively shorten the fibers, and one or more breaking zones that set the average fiber length and set the variability of fiber length (% CV). The sliver formed may be processed in an entwining mechanism (to facilitate subsequent handling), heat treated, and collected in a canister. It is expected that the sliver would be further processed, as in a spinning machine, to produce small denier yarns. The process handles feed tows of 3.0 denier per filament and 110,000–220,000 denier, and in a band having a width greater than 270 mm in the drawing and breaking zones. In the example illustrated in FIG. 1, a first preliminary breaking zone, zone II, is at least 500 mm long and the filament lengths resulting from this zone have a “nearly normal distribution” of fiber lengths between a few millimeters and the length of zone II. The zone II length is an optimization between a longer length, which reduces the breaking forces, and a shorter length, which avoids floc breaks and improves operating conditions. There is a second preliminary breaking zone, zone III, which is at least 200 mm and less than 1000 mm which is “considerably shorter” than zone II. There is then a first breaking zone, zone IV, which sets the average fiber length and appears shorter than zone III; and a second breaking zone, zone V, which eliminates overly long fibers, sets the variations in fiber length (characterized by % CV), and appears shorter than zone IV. In zone V, the “breaking distortions” (believed to be speed ratios) are at least 2× those in zone IV.
A horizontal in-line process for making a fasciated yarn from a tow of fibers is taught by Minorikawa et al in U.S. Pat. No. 4,667,463. The process involves drawing the tow over a heater in an elongated area having a narrow width, draft cutting the tow, and subjecting the draft cut fibers to an amendatory draft cutting step and a yarn formation step. The length of the zone in the amendatory draft cutting step is about 0.4 to 0.9 times the length of the draft cutting zone and the draw ratio for the amendatory draft cutting is at least 2.5×. The drawing preferably occurs in two stages to achieve a draw ratio of 90–99% of the maximum draw ratio and the drawn fiber is then heat treated. The yarn formation step uses a jet system for consolidating the fibers by creating wrapper fibers around the fiber core and wrapping them around the core fibers. Occasionally, apron bands are used in the amendatory draft cutting zone and yarn formation zone to regulate the peripheral fibers. The product is described in U.S. Pat. No. 4,356,690 to Minorikawa et al as being characterized by the fact that more than about 15% of the filaments in the yarn have a filament length of less than 0.5 times the average filament length of the yarn and more than about 15% of the filaments in the yarn have a filament length greater than 1.5 times the average filament length of the yarn. In the examples shown, the maximum output speed of the process making yarns of 174 to 532 denier (30.5 to 10 cotton count) is 200 meters/minute (ex. 6) with most examples run at about 100 meters/minute.
There is a problem with the products produced by Adams et al in that the 1.5–20% of the discontinuous filaments exceeding 76 cm in length that are produced in the single stretch-breaking zone cause problems in further processing (primarily roll wraps) especially if a non-vertical process orientation is chosen. There is also a problem with long filaments in the product of Adams in that it limits the number of filament ends that are available to protrude from the yarn and provide a yarn with a comfortable feel and look for textile applications.
In the case of Gilhaus' horizontal orientation, it may only be easily applied to processing large tows where it is believed the large number of filaments contribute to good intra-bundle friction between discontinuous filaments so bundle integrity can be maintained in the process without difficulty. In the case of Adams, the small numbers of filaments in the unconsolidated discontinuous yarn provide little frictional cohesion. A vertical orientation is believed required to eliminate lateral forces on the delicate yarn due to gravity before consolidation strengthens the yarn.
Adams proposes doing all stretch breaking in one zone and any drafting of the yarn in the same zone. Such a multipurpose zone makes independent optimization of final yarn parameters difficult or impossible.
Minorikawa et al may have a problem controlling discontinuous filaments as evidenced by the use of apron bands. This lack of control and the use of apron bands may limit the speed of his process to that disclosed in his examples which at 200 m/min is too slow for commercial production of a single low denier yarn line.
There is a need for an improved process for producing a stretch-broken yarn where the operating parameters can be independently optimized, where the process is not constrained to operate in a vertical orientation, and where excessively long filaments are not present that may separate from the filament bundle and wrap in the processing equipment and limit the number of filament ends in the yarn. There is a need for a process that can operate robustly and at a high speed above 250 m/min to make production of one yarn line at a time directly from tow economically attractive.